A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic projection apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source. A common feature of any plasma source is the production of fast ions and atoms, which are expelled from the plasma in all directions. These particles can be damaging to the collector and condenser mirrors which are generally multilayer mirrors or grazing incidence mirrors, with fragile surfaces. This surface is gradually degraded due to the impact, or sputtering, of the particles expelled from the plasma and the lifetime of the mirrors is thus decreased. The sputtering effect is particularly problematic for the radiation collector or collector mirror. The purpose of the collector is to collect radiation which is emitted in all directions by the plasma source and direct it towards other mirrors in the illumination system. The radiation collector is positioned very close to, and in line-of-sight with, the source of EUV in the plasma source and therefore receives a large flux of fast particles from the plasma. Other mirrors in the system are generally damaged to a lesser degree by sputtering of particles expelled from the plasma since they may be shielded to some extent.
In the near future, extreme ultraviolet (EUV) sources will probably use tin (Sn) or another metal vapor to produce EUV radiation. This tin may be deposited on mirrors, e.g. a mirror of the radiation collector, and/or leak into the lithographic apparatus. A mirror of such a radiation collector may have a EUV reflecting top layer of, for example, ruthenium (Ru). Deposition of more than approximately 10 nm tin (Sn) on the reflecting Ru layer may reflect EUV radiation in the same way as bulk Sn. The overall transmission of the collector would decrease significantly, since the reflection coefficient of tin is much lower than the reflection coefficient of ruthenium.
PCT Patent Application Publication No. WO 99/42904 discloses a filter that is, in use, situated in a path along which the radiation propagates away from the source. The filter may thus be placed between the radiation source and, for example, the illumination system. The filter includes a plurality of foils that, in use, trap debris particles, such as atoms and micro particles. Also, clusters of such micro particles may be trapped by these foils. These foils are oriented such that radiation can still propagate through the filter. The foils may be flat or conical and may be arranged radially around the path. The source, the filter and the projection system may be arranged in a buffer gas, for example, krypton at a pressure of about 0.5 torr.
PCT Patent Application Publication No. WO 03/034153 discloses a contaminant barrier that includes a first set of foils and a second set of foils, such that radiation leaving the source first passes the first set of foils and then the second set of foils. The foils of the first and second set define a first set of channels and a second set of channels, respectively. The two sets of channels are spaced apart leaving between them a space into which flushing gas is supplied by a gas supply. An exhaust system may be provided to remove gas from the contaminant barrier.
European Patent Application Publication No. EP 1 434 098 provides a contaminant barrier that includes an inner ring and an outer ring in which each of the foils is slidably positioned at least one of its outer ends in grooves of at least one of the inner ring and outer ring. By slidably positioning one of the outer ends of the foils, the foils can expand in a radial direction. The contaminant barrier may include a cooling system arranged to cool one of the rings to which the foils are thermally connected.
In order to prevent debris from the source or secondary particles generated by this debris from depositing on an optical element, a filter device may be used, such as for instance described in United States Patent Application Publication No. US 2006/0186353.